The next generation of networks will integrate services for different kinds of applications: delay-insensitive asynchronous applications like fax, mail, and file transfer, and delay-sensitive applications with real-time requirements, such as audio and video. These different applications have traditionally been supported by different network technologies. Asynchronous communication has been provided by computer networks, which are packet-switched and use store-and-forward techniques, like the Internet. Real-time communication, on the other hand, has been provided by circuit switched, time-division multiplexed telephone networks.
Circuit-switched networks have many attractive properties. Circuits are isolated from each other in the sense that traffic on one circuit is unaffected by activities on the others. This makes it possible to provide guaranteed transfer quality with constant delay, which is suitable for applications with timing requirements. Furthermore, data and control are separated in circuit-switched networks. Processing of control information only takes place at establishment and tear-down of circuits, and the actual data transfer can be done without processing of the data stream, congestion control, etc. This allows large volumes of data to be transferred efficiently. We think that this will be even more important in the future, since developments in photonics will dramatically reduce the cost of transmission, and switches will become the main communication bottlenecks.
The static nature of ordinary circuit-switched networks makes them inappropriate for certain types of traffic. Traditionally, circuits have fixed capacity, long set-up delay and poor support for multicast. These shortcomings make it difficult to efficiently support, for example, computer communication in a circuit-switched network. This has motivated a search for alternative solutions, and the predominant view is that the next generation of telecommunication networks should be cell-switched, based on ATM. Cells are small, fixed-size packets, so this is an orientation towards packet-switching. This means that many of the weaknesses of packet-switching are present also in cell-switched networks, in particular in the area of providing guaranteed quality of service. Therefore additional mechanisms, such as admission control, traffic regulation, scheduling of packets on links and resynchronization at the receiver are needed to integrate support for different kinds of traffic. One of the main concerns with packet switched networks in general, and ATM in particular, is whether it is possible to realize these mechanisms in a cost-effective way.
DTM, Dynamic synchronous Transfer Mode, is a broadband network architecture developed at the Royal Institute of Technology in Sweden. It is an attempt to combine the advantages of circuit-switching and packet-switching, in that it is based on fast circuit-switching augmented with dynamic reallocation of resources, supports multicast channels, and has means for providing short access delay. The DTM architecture spans from medium access, including a synchronization scheme, up to routing and addressing of logical ports at the receiver. DTM is designed to support various types of traffic and can be used directly for application-to-application communication, or as a carrier network for other protocols; such as ATM or IF. A prototype implementation based on 622 Mbps optical fibers has been operational for two years, and work is in progress with a wavelength-division multiplexed version with four wavelengths.
Fast circuit-switching was proposed for use in telephone systems already in the early 1980s. A fast circuit-switched telephone network attempts to allocate a transmission path of a given data rate to a set of network users only when they are actively transmitting information. This means that a circuit is established for every burst of information. When silence is detected, the transmission capacity is quickly reallocated to other users. In the form used in TASI-E, fast circuit-switching has been deployed for intercontinental communication between Europe and the United States. Burst switching is another form of fast circuit-switching, where a burst (consisting of a header, an arbitrary amount of data and a termination character) is sent in a time-division channel of fixed bit-rate and is thus interleaved with other bursts. This makes burst switching different from fast packet switching, where packets are sent one at a time with full link bandwidth. Furthermore, the length of a burst is not, in contrast to a packet, determined before the start of transmission.
It has been shown that the signaling delay associated with creation and tear-down of communication channels determines much of the efficiency of fast circuit-switching. DTM is therefore designed to create channels fast, within a few hundreds of microseconds. DTM differs from burst switching in that control and data are separated, and it uses multi-cast, multi-rate, high capacity channels to support a variety of different traffic classes. This means for example that it is possible to increase or decrease the allocated resources of an existing channel. Even though a DTM network may have the potential to create a channel for every message, we do not believe this approach suitable for all traffic classes. Rather, it is up to the user to decide whether to establish a channel per information burst or to keep the channel established even during idle periods.
New high-capacity communication networks and protocols are constantly being developed by the communications industry and academia. This development changes frequently and new results are important to application developers who integrate real-time audio, video, and asynchronous communication services into applications. The applications can be found on a wide range of network access terminals. Terminals act as network hosts and may be almost any electronic device, including small pocket telephones or television sets, multi-media work-stations, and million-dollar super-computers. Hosts differ by orders of magnitude in their needs for processing power, and in their communication services requirements. These disparate needs are currently reflected in a set of independent network classes. Each network class is optimized for its particular traffic and applications: cable television networks use unidirectional broadcast networks where the capacity is divided into fixed-sized sub-channels carrying video. A telephone network uses only 64 kbit/s duplex circuits with guaranteed throughput and tightly controlled delay variations. Computer networks, such as the Internet, allow a large number of parallel network sessions by use of connectionless packet switching. They also use statistical multi-plexing to efficiently utilize links. A backbone network for mobile systems needs extra control (or signaling) capacity to dynamically track all active terminals.
With this wide spectrum of applications existing today and to be augmented in the future, it is infeasible to continually invent new, sometimes global, networks and a new terminal interface for each new type of service. Instead, an integrated services network supporting existing and new services needs to be deployed. The overall goals for such a network are scaleability to global size and maximum sharing of expensive network components to minimize cost. Optical transmission technology has been shown to provide the necessary link capacity at a low enough price to make integrated services networks a realistic solution.
A new integrated optical network with much higher capacity will, however, introduce new problems not seen in today's more specialized and lower performance networks. First, when network capacity increases and the information flight latency remains limited by the speed of light, the increasing bandwidth delay product will put higher demands on mechanisms that isolate a user's traffic from third-party network traffic. A telephone session, for example, should not be affected by another user opening a high-capacity video channel. Second, applications and protocols will need to operate reliably with an increasing amount of information in transit to benefit from the increased network capacity. This will lead to larger burst and transaction sizes in the network.
Current networks using stateless packet switching protocols such as the Internet Protocol (IP)[rfc791,Come91:Internetworking1] have turned out to be extremely scalable. They have evolved from small networks connecting just a few Defense Advanced Research Projects Agency (DARPA) [rfc1120,Come91:Internetworking1] research computers in the mid seventies to the current global and ubiquitous Internet[rfc1118].
Shared medium local area networks (see [stallings94:data]) (LAN's) such as CSMA/CD, token ring and FDDI are used in the Internet as simple building blocks connected by routers or bridges. The combination of easy expansion, low incremental node cost and tolerance to faulty nodes has resulted in simple, flexible, and robust networks. Also, the shared medium allows efficient application of new multicast protocols such as IP multicast[rfc988].
A drawback of the shared medium is that it typically permits only a single terminal to transmit at any time, thereby not utilizing all network segments efficiently. A scheme allowing the capacity of the medium to be reused may be designed, but this is often at the cost of complexity in the high-speed access control hardware. Access control mechanisms for a shared medium also depend strongly on the size of the network and are usually efficient only for local area environments.
The two main types of network are connection oriented circuit-switched networks used for telephony, and connectionless packet-switched networks exemplified by the Internet. When a circuit-switched network is used for data communication, circuits need to remain open between bursts of information, wasting network capacity. This problem arises because circuit management operations are slow compared to dynamic variations in user demand. Another source of overhead in circuit-switched networks is the limitation of requiring symmetrical duplex channels, which introduce 100% overhead when information flow is unidirectional. This constraint also makes multicast circuits inefficient and difficult to implement. A connectionless packet-switched network, on the other hand, lacks resource reservation and must add header information to each message before transmission. Furthermore, latency in a connectionless packet-switched network cannot be accurately predicted and packets may even be lost due to buffer overflow or corrupted headers. The latter two factors make real-time service difficult to support. Congestion avoidance mechanisms can isolate traffic streams of different users. These schemes are, however, limited to operating on a time scale comparable to the round-trip packet delay.
To address the problems mentioned above, the communications industry is focusing on the development of Asynchronous Transfer Mode (ATM). ATM has been proposed for LAN's and many future public networks. International Telegraph and Telephone Consultative Committee (CCITT) also adopted it to be used as the transfer standard in broadband ISDN (B-ISDN). ATM networks are connection-oriented and establish a channel just as circuit-switched networks, but use small fixed-sized packets called cells for information transfer. The packet-switched nature of ATM means that the network needs to have new mechanisms such as buffer resource managers and link schedulers to establish real-time guarantees for a connection.
Our solution for providing real-time guarantees centers on a circuit-switched network and we must therefore address circuit-switching concerns such as those described above. We also employ a new shared-medium control protocol, and so must consider common shared-medium problems. Our design, called Dynamic synchronous Transfer Mode (DTM), uses channels as the communication abstraction. Our channels differ from telephony circuits in various ways. First, establishment delay is short so that resources can be allocated/deallocated dynamically as fast as user requirements change. Second, they are simplex and so minimize overhead when the communication is unidirectional. Third, they offer multiple bit-rates to support large variations in user capacity requirements. Finally, they are multicast, allowing any number of destinations.
DTM channels share many beneficial properties with circuits. There is no transfer of control information after channel establishment, resulting in very high utilization of network resources for large data transfers. Support of real-time traffic is natural; there is no need for policing, congestion control or flow-control within the network. The control information is separated from data, which makes multicast less complex. The transmission delay is negligible (i.e. less than 125 us) and there is no potential for data loss caused by buffer overflow as in ATM. Bit-error rates depend on the underlying link technologies, and switches are simple and fast due to strict reservation of resources at channel setup.
The present invention relates to a method and arrangement for centralized and distributed management of capacity (communication resources) in a circuit switched network with shared medium topology such as bus or ring topology of DTM type (digital synchronous transmission) in which the network use the capacity which are divided into cycles which in turn are divided into control time slots for signaling and data time slots for data transmission. Each data time slot is associated with a token. In the centralized version is a first node, called server node, that is assigned tokens corresponding to the unidirectionally flowing data time slots on the communication link (e.g. bus or ring). A second node requests tokens corresponding to a certain capacity from the server node and the server node reserves and transfers tokens corresponding to the requested capacity to the other node if the server node has the requested capacity available. In the distributed version, the number of server nodes is between two and the total numbers of nodesconnected to the shared medium topology.